1. Field of the Invention
This invention relates to large electrodynamic machines, such as electrical motors and generators and, more specifically, to a rotor assembly end turn cooling system and method for cooling the end turn region of coil-wound rotor assemblies in gas-cooled electrical generators.
2. Description of the Related Art
Large gas-cooled electrical generators are designed to have the highest power density possible while also being simple to manufacture. As the power density of an electrical generator increases, however, more heat is produced in the stator and rotor windings during normal operation. The need to conduct waste heat away from the stator and rotor windings thus operates as a significant constraint to the power densities that can be achieved.
Traditionally, the end turn regions of strap-wound rotor windings have been difficult to effectively cool. Examples of the present state of the art are shown in FIGS. 1 and 2 wherein the end turns and cooling passages are not shown so as to simplify illustration inasmuch as such structures are known to those skilled in the art. In current end turn cooling designs, the cooling fluid, for example air or hydrogen, is drawn over a cylindrical baffle shell 67 as shown by arrows 85 into cooling fluid passages (not shown) between end turns (not shown) from the high pressure zones 70 (source zones) comprising part of the annular space 21 between the baffle shell 67 and the rotor shaft extension 13. After being drawn through the cooling passages in or around the end turns (not shown), the cooling fluid is drawn radially inwardly through exhaust openings 68 into either low pressure zones 71 (suction zones, see FIG. 1) or axial, low-pressure ducts 74 (FIG. 2). On its journey from the source zone 70, over the shell 67 through the end turns and into suction zone 71 or low-pressure duct 74, the cooling fluid may pass along the sides of the coil end turns through ventilation channels (not shown) cut into fiberglass blocking (not shown) located between the coil end turns, through passageways designed within each copper coil, or through “zig-zag” slots cut at angles through a stack of copper coils.
From either the low pressure zone 71 or axial duct 74, the cooling fluid is then directed into the rotor pole slots 75 and pumped out into the rotor-stator air gap either directly or when slot covers 77 are used, through openings 76 therein.
The blocks 73 (FIG. 1) that form the barriers between the high pressure zones 70 and low pressure zones 71 are typically difficult to fit, as the fit must be tight enough to minimize leaks while leaving sufficient clearances for assembly. Normally, grooves are machined into the surface of the rotor shaft extension 13 to locate the blocks 73, adding cost and complexity to the rotor design. Sealing the low pressure zones 71 on the face toward the end of the rotor requires additional end-cap blocks (not shown) that must be attached to the rotor end ring, requiring that stress-concentrating holes be included in the rotor end ring design. The wall blocks 73 also require fasteners and other attaching/locating/sealing hardware, all of which add to the cost and reduce the reliability of the overall generator design.
As shown in FIGS. 1-2, the prior art end turn cooling devices provide only limited locations for the inlet and exhaust of cooling fluid to and from the end turn cooling passages. Exhaust must occur either into the narrow low-pressure zones 71 formed by the cylindrical shell 67 and wall blocks 73, or into simple axial ducts 74 defined by the underside of the shell 67. Both the low pressure zones 71 and the linear ducts 74 have traditionally been limited to locations along the centerline 50 of the rotor assembly to match up with the pole slots 75.
It would be desirable, therefore, to provide a rotor assembly end turn cooling system and method in which wall blocks are not used to divide the annular space between the cylindrical shell and the rotor shaft extension into high and low pressure zones.
It would be desirable further to provide a rotor assembly end turn cooling system and method where cooling fluid enters each cooling passage at a plurality of locations and spent cooling fluid is exhausted from the passages through openings located in close proximity to the corners of the end turns to enhance cooling of the end turns by providing shorter cooling passages to be traversed by the cooling fluid.
It would be desirable further to provide a rotor assembly end turn cooling system employing a generally cylindrical baffle shell having helical exhaust ducts that allow spent cooling fluid to be exhausted from the passages through openings located in close proximity to the corners of the coil end turns to allow the use of the shorter cooling fluid passages.
Moreover, it would also be desirable to provide a rotor assembly end turn cooling system wherein each helical exhaust duct has a plurality of exhaust channels to control the amount of suction directed to each passageway or section thereof to control the rate of fluid flow therethrough.
It would be desirable additionally to provide a rotor assembly end turn cooling system wherein the size, depth and cross-section of each channel of the helical exhaust duct is selected to provide constant velocity of the spent cooling fluid along the length of the helical exhaust duct to reduce fluid flow resistance and thereby further improve the cooling of the end turns.
Further still, it would be desirable to provide a rotor assembly end turn cooling system that allows cooling fluid inlet and exhaust openings to be positioned at any desired locations along the cooling fluid passages disposed among the coil end turns.
Additionally, it would be desirable to provide a rotor assembly end turn cooling system having a baffle shell with a smooth inner surface to minimize gas pressure variation and flow disturbances within the annular space between the baffle shell and the rotor shaft extension.
The present invention in its various preferred embodiments described herein provides numerous improvements and benefits over the prior art rotor assembly end turn cooling systems and methods.